This invention relates to the art of thermal heads for thermal recording which are used in various types of printers, plotters, facsimile, recorders and the like as recording means.
Thermal materials comprising a thermal recording layer on a substrate of a film or the like are commonly used to record images produced in diagnosis by ultrasonic scanning (sonography).
This recording method, also referred to as thermal recording, eliminates the need for wet processing and offers several advantages including convenience in handling. Hence in recent years, the use of the thermal recording system is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is under review.
As is well known, thermal recording involves the use of a thermal head having a glaze, in which heating elements comprising heat-generating resistors and electrodes, used for heating the thermal recording layer of a thermal material to record an image, are arranged in one direction (main scanning direction) and, with the glaze urged at small pressure against the thermal material (thermal recording layer), the two members are moved relative to each other in the auxiliary scanning direction perpendicular to the main scanning direction, and energy is applied to the heating elements of the respective pixels in the glaze in accordance with image data to be recorded which were supplied from an image data supply source such as MRI in order to heat the thermal recording layer of the thermal material, thereby accomplishing image reproduction.
A protective film is formed on the surface of the glaze of the thermal head in order to protect the heating elements and the like for heating a thermal material. Therefore, it is this protective film that contacts the thermal material during thermal recording and the heating elements heat the thermal material through this protective film so as to perform thermal recording.
The protective film is usually made of wear-resistant ceramics; however, during thermal recording, the surface of the protective film is heated and kept in sliding contact with the thermal material, so it will gradually wear and deteriorate upon repeated recording.
If the wear of the protective film progresses, density unevenness will occur on the thermal image or a desired protective strength can not be maintained and, hence, the ability of the film to protect the heating elements is impaired to such an extent that the intended image recording is no longer possible (the head has lost its function).
Particularly in the applications such as the aforementioned medical use which require multiple gradation images of high quality, the trend is toward ensuring the desired high image quality by adopting thermal films with highly rigid substrates such as polyester films and also increasing the setting values of recording temperature (energy applied) and of the pressure at which the thermal head is urged against the thermal material. Under these circumstances, as compared with the conventional thermal recording, a greater force and more heat are exerted on the protective film of the thermal head, making wear and corrosion (or wear due to corrosion) more likely to progress.
With a view to preventing the wear of the protective film on the thermal head and improving its durability, a number of techniques to improve the performance of the protective film have been considered. Among others, a carbon protective film with carbon as a main component, that is, a carbon-based protective film (hereinafter referred to as a carbon protective layer) is known as a protective film excellent in resistance to wear and corrosion.
Thus, Examined Published Japanese Patent Applications (KOKOKU) No. 61-53955 and No. 4-62866 (the latter being the divisional application of the former) disclose a thermal head excellent in wear resistance and response which is obtained by forming a very thin carbon protective layer having a Vickers hardness of 4500 kg/mm.sup.2 or more as the protective film of the thermal head and a method of manufacturing the thermal head, respectively. The carbon protective layer has properties quite similar to those of diamond including a very high hardness and chemical stability, hence the carbon protective layer presents sufficiently excellent properties to prevent wear and corrosion which may be caused by the sliding contact with thermal materials.
The carbon protective layer is excellent in wear resistance, but brittle because of its hardness, that is, low in tenacity and rather inferior adhesion to a lower layer so that heat shock and thermal stress due to heating of heating elements may bring about rather easily cracking or peeling.
In order to solve the problem, Unexamined Published Japanese Patent Application (KOKAI) No. 7-132628 discloses a thermal head which has a dual protective film comprising a lower protective layer composed of a silicon-based compound and an upper carbon protective layer (diamond-like carbon layer), whereby the potential wear and breakage of the protective film due to heat chock and the like are significantly reduced with the help of enhanced adhesion of the carbon protective layer to ensure that high-quality images can be recorded over an extended period of time. In this document, another technique is disclosed that enhances the adhesion between the carbon protective layer and the lower protective layer by processing the surface of the lower protective layer employing a plasma-assisted CVD (chemical vapor deposition) in a reducing atmosphere.
However, the adhesion of the carbon protective layer is not enough and, especially, in the medical applications as described above, the carbon protective layer is liable to be cracked or peeled off by a stress due to a difference in coefficient of thermal expansion between the respective layers, a mechanical impact due to a foreign matter entered between the thermal material and the thermal head (glaze) during recording operation or other factors.
Cracking or peeling in the protective layer gives rise to wear, corrosion and wear due to corrosion, which results in reduction of the durability of the thermal head. The thermal head is not capable of exhibiting high reliability over an extended period of time.